Is it true that the AI is very passive and cheats? This is one of my biggest worries about this game - that it'll just be too easy because of the AI's inactiveness.

And also, how's the replayability of this game? Is it worth getting at this very moment?

Sorry if its not a right place to post this kinda stuff. This game looks insane and I'd love to buy it, but these are the only two questions that are stopping me from doing it :)

Everything is subjective so I won't even try to answer your question. I do have two other points to make: 1) there's a free demo that will let you play the first 20 turns: https://www.gog.com/en/game/emperor_of_the_fading_suns_enhanced_demo

2) The main game only costs $4.50 USD.

Oh, didn't know that! Thanks for letting me know.

The AI does not use resources. All of their units are produced for free. That extends to food to feed their units & cities. This was to compensate for the AI's inability to plan an economy back when the game was released 30 years ago. Whether that will remain the case going forward remains to be seen. So if you disable the universal warehouse (meaning planets will only have access to resources on, or in orbit of, that planet), be aware that you're only setting a personal challenge. I'd advise keeping the warehouse on for your first game or two.

I'm surprised no AI mod would have been made for it? They exist for games going back as far as Master of Magic.